Never seen that before. I'm looking into it. I know that the repository was throwing 404 errors earlier when you attempt to download.

After clicking a couple of times on the "Download Version 1" on this page http://wordpress.org/extend/plugins/wp-admin-menu/ , I was able to download and manually install the plugin. Maybe the 404 errors have something to do with your fopen() errors as there are no references to fopen() in the plugin itself.